Re: [PATCH] KVM: arm64: arch_timer shouldn't assume the vcpu is loaded

["André Przywara" <andre.przywara@arm.com>]

On 08/04/2020 11:07, Marc Zyngier wrote:

Hi Marc,

> Hi James,
> 
> Thanks for looking into this.
> 
> On Mon,  6 Apr 2020 16:03:55 +0100
> James Morse <james.morse@arm.com> wrote:
> 
>> kvm_arch_timer_get_input_level() needs to get the arch_timer_context for
>> a particular vcpu, and uses kvm_get_running_vcpu() to find it.
>>
>> kvm_arch_timer_get_input_level() may be called to handle a user-space
>> write to the redistributor, where the vcpu is not loaded. This causes
>> kvm_get_running_vcpu() to return NULL:
>> | Unable to handle kernel paging request at virtual address 0000000000001ec0
>> | Mem abort info:
>> |   ESR = 0x96000004
>> |   EC = 0x25: DABT (current EL), IL = 32 bits
>> |   SET = 0, FnV = 0
>> |   EA = 0, S1PTW = 0
>> | Data abort info:
>> |   ISV = 0, ISS = 0x00000004
>> |   CM = 0, WnR = 0
>> | user pgtable: 4k pages, 48-bit VAs, pgdp=000000003cbf9000
>> | [0000000000001ec0] pgd=0000000000000000
>> | Internal error: Oops: 96000004 [#1] PREEMPT SMP
>> | Modules linked in: r8169 realtek efivarfs ip_tables x_tables
>> | CPU: 1 PID: 2615 Comm: qemu-system-aar Not tainted 5.6.0-rc7 #30
>> | Hardware name: Marvell mvebu_armada-37xx/mvebu_armada-37xx, BIOS 2018.03-devel-18.12.3-gc9aa92c-armbian 02/20/2019
>> | pstate: 00000085 (nzcv daIf -PAN -UAO)
>> | pc : kvm_arch_timer_get_input_level+0x1c/0x68
>> | lr : kvm_arch_timer_get_input_level+0x1c/0x68
>>
>> | Call trace:
>> |  kvm_arch_timer_get_input_level+0x1c/0x68
>> |  vgic_get_phys_line_level+0x3c/0x90
>> |  vgic_mmio_write_senable+0xe4/0x130
>> |  vgic_uaccess+0xe0/0x100
>> |  vgic_v3_redist_uaccess+0x5c/0x80
>> |  vgic_v3_attr_regs_access+0xf0/0x200
>> |  nvgic_v3_set_attr+0x234/0x250
>> |  kvm_device_ioctl_attr+0xa4/0xf8
>> |  kvm_device_ioctl+0x7c/0xc0
>> |  ksys_ioctl+0x1fc/0xc18
>> |  __arm64_sys_ioctl+0x24/0x30
>> |  do_el0_svc+0x7c/0x148
>> |  el0_sync_handler+0x138/0x258
>> |  el0_sync+0x140/0x180
>> | Code: 910003fd f9000bf3 2a0003f3 97ff650c (b95ec001)
>> | ---[ end trace 81287612d93f1e70 ]---
>> | note: qemu-system-aar[2615] exited with preempt_count 1
>>
>> Loading the vcpu doesn't make a lot of sense for handling a device ioctl(),
>> so instead pass the vcpu through to kvm_arch_timer_get_input_level(). Its
>> not clear that an intid makes much sense without the paired vcpu.
> 
> I don't fully agree with the analysis, Remember we are looking at the
> state of the physical interrupt associated with a virtual interrupt, so
> the vcpu doesn't quite make sense here if it isn't loaded.

But wasn't it that this function is meant to specifically deal with this
*without* going to the hardware (which is costly, hence this
optimisation)? Because for the timer we *can* work out the logical IRQ
line state by examining our saved state? And this is what we do in
kvm_timer_should_fire(), when timer_ctx->loaded is false.

Which for me this sounds like the right thing to do in this situation:
the VCPU (and the timer) is not loaded, so we check our saved state and
construct the logical line level. We just need a valid VCPU struct to
achieve this, and hope for the virtual timer to be already initialised.

Do I miss something here?

Also to me it sound like the interface for this function is slightly
lacking, because just an intid is not enough to uniquely identify an
IRQ. It was just fine so far because of this special use case.

Cheers,
Andre

> 
> What does it mean to look at the HW timer when we are not in the right
> context? For all we know, it is completely random (the only guarantee
> we have is that it is disabled, actually).
> 
> My gut feeling is that this is another instance where we should provide
> specific userspace accessors that would only deal with the virtual
> state, and leave anything that deals with the physical state of the
> interrupt to be exercised only by the guest.
> 
> Does it make sense?
> 
> Thanks,
> 
> 	M.
> 

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